It is a common misconnception that these sounds are produced in the alveoli. They are not. The velocity of air in the alveoli is not significant enough to produce turbulance and sounds.

• The breath sound that is normally produced at the trachea is bronchial.
• The breath sound that is normally heard on the chest wall, respiratory areas, is vesicular.

Now, here is the explanation -

The bronchial breath sounds produced at the major airways have to travel all across the tissues (bronchi, bronchioles, alveolar walls, blood vessels, ribs, muscles, subcutaneous tissue, skin) to reach the body surface from where they can be auscultated.

While they are being transmitted through these tissues, some frequencies of sound are absorbed and the character of the sound changes. This changed sound is termed vesicular breath sound.

• Here, inflammatory exudates would have filled the alveoli, thereby replacing the usual air. Inflammatory exudate is liquid to semisolid in consistency.
• Liquid & Solids are better conductors of sound than air.
• As air is replaced by a better conducting medium, the bronchial sound produced at the trachea is conduced better and transmitted as it is.
• Therefore, we hear bronchial breath sound on an area of consolidation.

• In both the above conditions, as there is loss of lung volume, the traches is pulled towards the side of lesion. When the trachea is pulled, the distance between the trachea and the chest wall reduced on the affected side,.
• The sound has to travel for a lesser distance than earlier, there is less filtration of frequencies, which ensures that the sound remains bronchial.

• When a lung cavity is connected to a bronchiole, bronchial sound is produced in the cavity, due to air entering a hollow chamber.
• To understand how this happens, take an empty bottle and blow gently over the top of the open bottle. You will hear a 'hollow' sound. Thsi is the characted of bronchial breathing.
• If the patient also has fibrosis in adition to the cavity, a common combination in post tubercular lesions, fibrosis also contributes to bronchial breathing.

Upper level of pleural effusion

• In pleural effusion, fluid generally accumulates below upwards. As it accumulates, the lung tissue is compressed and displaced upwards.
• This compressed lung tissue is of a higher density than earlier and can conduct sounds better.